Continuous process for the air oxidation of sour water

ABSTRACT

A CONTINUOUS PROCESS FOR THE LIQUID PHASE, NON-CATALYTIC, AIR OXIDATION OF SOUR WATER IN WHICH THE SULFUR COMPOUNDS IN THE WATER ARE CONVERTED TO NON-POLLUTING, NONOXYGEN DEMANDING COMPOUNDS INVOLVES HEATING THE WATER AND OXIDIZING THE HEATED WATER WITH COUNTERCURRENT OR CO-CURRENT FLOW OF THE WATER AND OXIDIZING MEDIUM. THE PROCESS RAPIDLY CONVERTS OBJECTIONABLE SULFIDES AND INTERMEDIATE SULFUR COMPOUNDS SUCH AS THIOSULFATES, TETRATHIONATES, POLYTHIONATES, SULFITES AND POLYSULFIDES TO SULFATE.

Sept. 25, 1973 D. E. M COY E AL CONT INUOUS PROCESS FOR THE AIR OXIDATION OF SOUR WATER l8 EFFLUENT 2 WATER FEED-EFFLUENT Filed Oct. 6, 1971 FIG/I FEED WATER HEATER HEAT EXCHANGER -l6 OFF-GAS 2 ;44

{AIR-WATER SEPARATOR wATER EFFLUENTS 42 46 FEED f WATER 34 FEED WATER HEATER FEED-EFFLUENT 62 HEAT EXCHANGER OXYGEN OFF GAS OXIDQIZER TOWER AIR COMPRESSOR 24 AIR OR OXYGEN United States Patent 3,761,409 CONTINUOUS PROCESS FOR THE AIR OXIDATION OF SOUR WATER Drew E. McCoy, Robert M. McEachern, and Roger M. Dille, Richmond, Va., assignors to Texaco Inc., New York, NY.

Filed Oct. 6, 1971, Ser. No. 186,893 Int. Cl. C02c 5/04 US. Cl. 210-63 9 Claims ABSTRACT OF THE DISCLOSURE A continuous process for the liquid phase, non-catalytic, air oxidation of sour water in which the sulfur compounds in the water are converted to non-polluting, nonoxygen demanding compounds involves heating the water and oxidizing the heated water with countercurrent or co-current flow of the water and oxidizing medium. The process rapidly converts objectionable sulfides and intermediate sulfur compounds such as thiosulfates, tetrathionates, polythionates, sulfites and polysulfides to sulfate.

The invention relates generally to air and water pollution abatement, and more particularly to a continuous, liquid phase, air oxidation process for converting sulfur compounds present in water to non-polluting, non-oxygen demanding sulfates.

As is well known, water which contains hydrogen sulfide and other intermediate sulfur compounds such as thiosulfate, tetrathionate, polythionate and polysulfide originates from many sources. A number of processes used in refining petroleum produce Water efiiuents which contain high concentrations of hydrogen sulfide and ammonia. These waters are typically called foul or sour water. Typical operations producing sour water include such widely practised processes as crude distillation, hydrotreating, catalytic cracking, thermal cracking, delayed coking and hydrocracking. Ammonia also is usually present in sour water streams either because it has been added to neutralize the H 8 for corrosion control or as the result of hydrogenation of nitrogen during the refining process. The H S and ammonia react to form ammonium sulfide and ammonium hydrosulfide, depending on the pH of the water and, if free sulfur is present, polysulfides. Normally the pH of the refinery sour water is approximately 9.0 and the sulfides are present as hydrosulfide ion HS-.

Sulfur is also present in waters used in the production of cellulose pulp for paper where sulfur compounds help to dissolve the lignin from wood fibers. The sulfate or Kraft process, one of the most widely processes for producing paper pulp, uses a White pulping liquor containing about 30 percent by weight of sulfide (present mainly as sodium sulfide and sodium hydrosulfide). After the pulp has been digested with the sulfide liquor, the liquor (termed black liquor) is washed from the pulp and sent to a recovery process. Some sulfur compounds such as sulfide and sulfite are unavoidable missed by the recovery process and end up in effluent from the pulping plant.

In some instances small amounts of H S, sometimes in the presence of S0 are emitted from a process such as a coking oven or sulfur recovery plant. These H 8 and S0 gases can be scrubbed from the gas using sodium hydroxide or some other strongly basic scrubbing liquor. Recovery of by-product sulfur compounds from these small streams is usually not economical and some acceptable means of disposing of the scrubbing liquor is needed.

A number of reasons exist for removing sulfides and intermediate sulfur compounds from water. Hydrogen sulfide is objectionable because of its toxicity and its very 'ice unpleasant odor but, more importantly, because it exerts a very high BOD and COD on the receiving waters with well known attendant results. The intermediate sulfur compounds such,as thiosulfate and polythionate are not as objectionable as the sulfide, but they still exert a BOD and COD on the receiving waters. Sulfate is the only sulfur species, which does not exert a COD on the receiving water.

Up to now, the most comonly used method for treating sulfide-containing waters is to use some type of stripping medium such as steam, air, natural gas, etc., to strip H S from the water. In the past, most of the H 8 from the stripping towers was routed to burners or lflares where the H 8 was converted to S0 and released to the atmosphere. In a petroleum refinery, sour Water stripping removes NH as well as H 5. However, combustion of this stream can result in the production of nitrogen oxides as well as S0 Air quality regulations have halted or severely restricted the discharge of S0 and oxides of nitrogen. In many instances, the H 8 from the stripping towers is sent to a sulfur recovery unit where the H 8 is converted to elemental sulfur. Most sulfur recovery units have restrictions which limit the amount of NH which can be present in the H S stream.

An oxidation process is currently used to convert sulfide into intermediate sulfur compounds, mainly thio sulfate. This process contacts air and sour water in a multi-chamber tower at 200 F. and 90 p.s.i.g. pressure. The thiosulfates produced by this process are not acceptable in many cases because of their high BOD and COD.

Catalytic oxidation of hydrogen sulfide is also known. Thus Snavely and Blount have described in Corrosion, vol. 25, page 297 (1969) a method of scavenging oxygen in water with hydrogen sulfide by using a transition metal catalyst. US. Pat. 3,576,738 teaches a process wherein sour gas, water, air and nickel chloride are pressurized to dissolve the air in the sour water followed by depressurizing to allow the air to come out of solution and cause a reaction between the oxygen and the hydrogen sulfide. Both of these methods are unsatisfactory from the economic standpoint. Aside from equipment costs, the only costs involved in the present process are the costs of the acid or base used to fix the pH of the sour water.

The present process chemically oxidizes sulfide, polysulfide, thiosulfate, tetrathionate and polythionate. The process comprises adjusting the pH of the stream to between about 6 and 13, preferably to between 8.0 and 12.5, heating the stream to a temperature ranging from 250 to 520 F., preferably 325 and 475 F., oxidizing the heated stream under a pressure of -800 p.s.i.g. and preferably 200 to 500 p.s.i.g. at a liquid hourly space velocity of between 0.5 to 12.0 volume of feed per reactor void volume, preferably of between 3 and 9 with an oxidizing medium containing from 0 to 500 percent excess oxygen basis stoichiometric conversion to sulfates and preferably 5 to 200 percent excess oxygen, the oxidation being carried out by counteror cocurrent flow of the stream and of the oxidizing medium. Optionally, air and water and off-gases formed during the oxidation step are separated from the oxidized efiluent.

In the drawing, FIG. 1 shows schematically an embodiment of the invention wherein oxidation is carried out on a countercurrent contact tower while FIG. 2 illustrates the process as carried out in a cocurrent contact tower.

Referring to FIG. 1, feed water, the pH of which has been adjusted as required, is flowed through line 10 to feed pump 12 which pumps it under pressure to feedefiluent heat exchanger 14 and thence to heater 20 and oxidizer tower 22.. Heater 20 can be of any conventional construction and can be fired or steam heated. The oxidizer tower can be packed with Raschig rings, berl saddles, pall rings, or other suitable packings, or can contain bubble or sieve trays. The tower serves to contact the hot sour water with the oxidizing medium air, oxygen or a mixture thereof in any proportion. This medium is introduced via line 24 and compressor 26 which brings it to systems pressure and then through line 28 in countercurrent relationship with the feed. The sulfur compounds react with the oxygen to form sulfate as shown in Equations 1 and 2 for sulfides and thiosulfates, respectively.

The amount of oxygen required for complete oxidation to sulfate will depend upon which sulfur compounds are present and their relative amounts. To oxidize sulfides, an oxygen to sulfur weight ratio of 2 is required to produce sulfate. Oxidation of thiosulfate requires V2 as much oxygen as do sulfides. An excess of oxygen is desirable to assure complete and rapid oxidation of the sulfur compounds. Unused oxygen and inerts in the gas stream pass out of the top of the tower through line 30. The water efiluent from the bottom of the oxidizer passes through line 16 and the feed eflluent heat exchanger and is discharged through line 18, thereby preheating the feed.

In FIG. 2 the process is illustrated as operating with cocurrent downfiow of oxidizing medium and water through the oxidizer tower. In this embodiment of the invention, sour water is fed through line 32 and mixed with the oxidizing medium supplied through line 34 before flowing into heat exchanger 36 where it is preheated to between 200 F. and 500 F. by contact with hot effluent arriving from the oxidizer tower through line 52. The preheated water is heated to operating temperature in heated 46 and passes into oxidizer tower 50 which preferably is packed with Raschig rings, berl saddles, pall rings or other suitable packing. The oxidized effluent still containing off-gases passes into air separator 40. The off-gas, mainly oxygen depleted air, is taken off through line 44 for discharge to the atmosphere or routed to a flue gas stack, and the water eflluent is taken olf through line 42. The water has a reduced BOD and COD and may be discharged safely to a receiving water depending upon the concentration of other contaminants in the water.

Both embodiments of the invention are particularly useful for treating scrubbing liquors from tail gas units for gasoline plants, refineries or other chemical processing plants which utilize tail gas scrubbing systems.

Following are specific examples of water treatments according to the embodiments illustrated in FIGS. 1 and 2, the analysis of the untreated sour water being given in Table I below:

Table I.Untreated sour water analysis S= 7,495 p.p.m. as S.

0 z 320 p.p.m. as S.

S 0 257 p.p.m. as S.

S0 44 p.p.m. as S.

NH 4,600 p.p.m. as N EXAMPLE 1 The analysis of the oxidized water after treating the sour water of Table I with 100% excess of stoichiometric air requirement for the oxidation of sulfide to sulfates at 500 F., 750 p.s.i.g. and 6.4 Liquid Hourly Space Velocity (LHSV) was the following:

4 EXAMPLE 2 The analysis of the oxidized water after treating the sour water of Table I with excess of stoichiometric air requirement for the oxidation of sulfides to sulfates at 450 F., 450 p.s.i.g. and 3.8 LHSV was the following:

SAMPLE NO. 2

S 0p.p.m. as S.

5 0 32 p.p.m. as S.

S O 0 p.p.m. as S.

SO 6,864 p.p.m. as S.

Total S 7,369 p.p.m. as S.

Percent conversion to SO 93%.

EXAMPLE 3 The analysis of oxidized water after treating the sour water of Table II below with 100% excess of stoichiometric air requirement for the oxidation of sulfides to sulfates at 400 F., 300 p.s.i.g. and 3.8 LHSV was the following:

EXAMPLE 4 The analysis of partially oxidized water after treating the sour water of Table II with 100% excess-of stoichiometric air requirement for the oxidation of thiosulfates to sulfates at 500 F., 750 p.s.i.g. and 5 LHSV was the following:

SAMPLE NO. 4

S= 0 p.p.m. as S. S O 96 p.p.m. as S. S40 0 p.p.m. as S. SO 7,411 p.p.m. as S. Total S 7,340 p.p.m. as S. Percent conversion to $0.;= 100%.

While the invention has been illustrated with physical embodiments, these are exemplary only and the scope of the invention is limited only by the subjoined claims.

We claim:

1. In a continuous process for the liquid phase air oxidation of water containing sulfide anion and at least one other sulfur containing anion selected from the group consisting of thiosulfates, tetrathionates, polythionates, sulfites and polysulfides to convert said to sulfites, the steps of: adjusting the pH of said water to between about 6 to 13, heating said water to a temperature ranging from about 250 to about 520 F.; oxidizing the heated water under a pressure of around 75 to 800 p.s.i.g. at a liquid hourly space velocity of between 0.5 to 12.0 volumes of feed per reactor void volume with a noncatalytic, oxidizing medium containing from 0 to 500% excess oxygen basis stoichiometric conversion of sulfide to sulfates and recovering an efiiuent containing substantially no other anion than said sulfate.

2. The process of claim 1 wherein said oxidation is carried out by countercurrent flow of said water and said oxidizing medium.

5 6 3. The process of claim 1 wherein said oxidation is 8. The process of claim 1 wherein said liquid hourly carried out by cocurrent flow of said water and said space velocity ranges between 3 and 9. oxidizing medium. 9. The process of claim 1 wherein said excess oxygen 4. The process of claim 3 wherein air and off-gases ranges from 5 200 P formed by said oxidation are separated from said efliuent. 5

5. The process of claim 1 wherein the pH of said FOREIGN PATENTS watenis brought to between 8.5 and 12.5. 1,074,391 11/ 1957 Germany 210-63 6. The process of claim 1 wherein said water is heated to a temperature of 325 to 475 F. MICHAEL ROGERS Pnmary Exammer 7. The process of claim 1 wherein said pressure ranges 10 Us, (1 X,

from 200 to 500 p.s.i.g. 162-46; 423-544 

